Adaptive resource use in a re-introduced black rhinoceros population

Abstract:

The aim of biological management for black rhinoceros (Diceros bicornis)
conservation is to maximise meta-population growth rates to aid species recovery. This
research investigated how adaptive resource use in response to seasonal variation in
resource availability could affect maximum productive habitat capacity for this critically
endangered species. Analysis was based on a population of rhinos which had shown
excellent annual growth rates and low inter-calving intervals since re-introduction to
Tswalu Kalahari Reserve in the Northern Cape of South Africa in 1995.
Acacia haematoxylon, a semi-evergreen species, was identified as the key
resource forming the majority of diet contents during the late dry season. Use of this
species resulted in a low level of seasonal variation in dietary contents of energy and
protein. During the data collection period, energy and protein gains of individual female
rhinos were estimated to exceed maximum requirements for reproduction throughout the
seasonal cycle. An experiment designed to test the compensatory growth response of A.
haematoxylon found that clipping trees in a way that simulated rhino browsing stimulated
an increased growth response in the following wet season. This response indicated
potential for a facilitatory relationship in the short term. A large proportion of the
available area at Tswalu was not used by black rhinos. Home range location and habitat
type selection within home ranges during the dry season were positively associated with
two shrubveld habitat types containing greater A. haematoxylon biomass than other
habitat types. Results from all aspects of field data analysis emphasized the importance
of A. haematoxylon as the key dry season resource for black rhino at Tswalu.
Field data were used to develop a conceptual model of how seasonally adaptive
resource use by black rhinos could determine maximum productive habitat capacity. The
crux of the model was to estimate the highest population density at which female rhinos
could attain maximum energetic gains for reproduction throughout the entire seasonal
cycle. The most limiting period was the nutritional bottleneck during the late dry season.
The rationale behind this approach was to enable females to maintain body condition and
be capable of meeting nutritional requirements for reproduction throughout the year, thus
minimizing inter-calving intervals and maximising population growth rates. Model
projections indicated that female rhinos could not attain energy gains for reproduction
throughout the dry season in certain habitat types due to low availability of A.
haematoxylon. However, model outputs indicated potential for an increase in rhino
density by approximately one third in the two favoured shrubveld habitat types, assuming
that habitat conditions remained unchanged.
Availability of semi-evergreen A. haematoxylon was identified as the key
vegetation component determining maximum productive habitat capacity for black rhino
at Tswalu. Monitoring available biomass of this species at the end of the dry season
could provide a simple plant-based indicator of how close the population is to maximum
productive habitat capacity. Managing rhino densities in fenced reserves elsewhere
around spatial and temporal availability of key resources may assist in achieving black
rhino conservation goals of maximising metapopulation growth rates.
Joanne Aileen Shaw